1.Field of the Invention
The present invention relates to a high-power optical fiber laser. More precisely, the invention relates to a fiber laser emitting at wavelengths in the blue band of the visible spectrum. The fiber laser allows generating a continuous or pulsed radiation in the nanosecond regime.
2.Description of the Related Art
Since its invention fifty years ago, the laser has experienced extraordinary developments, allowing having today lasers providing higher and higher energies or powers, in wavelength domains covering all the optical spectrum, and with shorter and shorter pulse durations reaching the femtosecond domain. Another axis of development aims to increase the integration of the laser systems, in particular by using compact sources, pump laser diodes and/or optical fiber amplifying mediums.
The 980 nm single-mode laser diodes available on the market have not only powers lower than the Watt in the continuous regime, but they do not allow reaching high peak powers.
Ytterbium-doped fiber lasers have been proposed as 980 nm laser sources. However, the 980 nm emission requires a very high pump fluence, due to the Ytterbium population levels. The emission of a laser beam of a power of 3.5 W has been demonstrated with a microstructured fiber having an air-clad and an Ytterbium-doped-guide to ensure the pump fluence level (K. H. Ylä-Jarkko et al., Advanced Solid-State Photonics, Vol. 83, OSA Trends in Optics and Photonics, 2003, paper 103). More recently, photonic fiber lasers have been developed, which comprise an Ytterbium-doped rod having a low ratio between core diameter and clad diameter, which allow obtaining a continuous 980 nm high-power (94 W) laser beam (J. Boullet et al., Opt. Express 16, 17891, 2008 and F. Roeser et al. Opt. Express 16, 17310, 2008). Nanosecond pulses of about 1 mJ have also been obtained with such photonic fibers. However, the use of photonic fibers poses manufacturing difficulties and do not allow manufacturing a compact and fully fibered laser system.
It is presently searched to develop robust and compact high-power lasers, operating in continuous or pulsed regime. It is searched to develop fiber lasers emitting around 980 nm, as well as in the blue part of the visible spectrum at powers of the order of the Watt. Such lasers find applications in many domains such as fluorescence spectroscopy, flow cytometry, biotechnologies, metrology, LIDARs and in particular the use thereof in bathymetry.
The present invention has for object to remedy these drawbacks and relates more particularly to a high-power optical fiber laser comprising an oscillator adapted to emit a source optical signal to be amplified, a pump laser adapted to emit a high-power pump optical radiation, a signal-amplifying optical fiber, adapted to receive said source optical signal and said high-power pump optical radiation. According to the invention, said pump laser comprises a plurality of multimode pump laser diodes and a laser cavity, said laser cavity comprising a double-clad fiber including a Neodymium-doped single-mode guide, an optical fiber Bragg grating forming an end of said laser cavity and a fiber reflector forming the other end of said laser cavity, said single-mode laser fiber being adapted to generate a laser radiation when it is optically pumped by a pump radiation coming from the plurality of pump laser diodes so that said laser cavity emits a high-power pump laser radiation, and said signal-amplifying optical fiber comprises a rare-earth-doped single-mode optical fiber section, so as to generate a high-power laser beam, when said signal-amplifying optical fiber is optically pumped by said high-power pump optical radiation.
According to particular aspects of the invention:                said single-mode optical fiber section of said signal-amplifying optical fiber comprises an Ytterbium-doped guide;        said signal-amplifying optical fiber has a high ion-doping rate so that the length of said single-mode optical fiber section of said signal-amplifying optical fiber is lower than a few tens of centimeters.        
According to a particular embodiment, the fiber laser comprises an optical frequency converter, said converter being adapted to receive said high-power laser beam coming from said signal-amplifying optical fiber in a wavelength band and to frequency-convert said high-power laser beam so as to generate said high-power laser beam in another wavelength band.
According to a particular aspect of this embodiment, said fiber laser further comprises an optical filtering means arranged between an output end of said signal-amplifying optical fiber and an input of said optical frequency converter, said optical filtering means being adapted to separate an optical radiation at the high-power pump wavelength from an optical radiation at the wavelength of said amplified optical signal.
According to other particular aspects of the invention, said fiber laser comprises:                an optical pump coupling means adapted to receive the pump laser radiations coming from said pump laser diodes and to couple said pump laser radiations to said pump-amplifying optical fiber;        optical coupling means adapted to couple the source optical signal to be amplified and said high-power pump optical radiation, respectively, in said signal-amplifying optical fiber;        said pump laser fiber is rare-earth-doped;        said pump laser fiber is a double-clad fiber having a leaky-mode refractive index profile and a rare-earth-doped core;        said oscillator comprises a fibered laser diode coupled to a pulse generator so as to generate high-power laser pulses.        